1. Field of the Invention
The present invention generally relates to the field of semiconductor wafer testing, and more particularly, to a non-contact method and apparatus for testing such wafers.
2. Discussion of the Related Art
There are a variety of important measurements that must be made on a semiconductor wafer to determine whether it is suitable for further device processing and to make process adjustments. Examples of such measurements include doping concentration measurements, charge time retention measurements, and general leakage measurements. However, present measurement techniques have repeatability problems, and those techniques that utilize metal-oxide-semiconductor (MOS) structures to make the measurements destroy the wafer under test.
One such measurement that must be made on a semiconductor wafer to determine whether it is suitable for further device processing is the monitoring of mobile ion contamination. Monitoring of mobile ion contamination, such as sodium or potassium, is very important for improved yield and reliability of field effect transistor (FET) products. Mobile ion contamination adversely affects the performance of an FET, in that, an amount of gate voltage needed to turn the FET device "ON" is modified by the presence of mobile ions in the gate oxide. Presently known techniques for monitoring mobile ion contamination include MOS current-voltage (I-V) loop and MOS pulsed charge-voltage (Q-V) measurements. With these techniques, they are especially sensitive to mobile ions, via the detection of ionic current transients during bias-temperature stressing. However, a disadvantage of these techniques is that they require the use of MOS structures. Using a MOS structure to detect ionic currents is expensive and time consuming, in terms of the sample preparation for MOS electrodes (i.e., aluminum dots) and the recycling of monitor wafers. Furthermore, MOS dot sample preparation increases the likelihood for introducing mobile ion contamination. Still further, mobile ions tend to get trapped at the aluminum dot/oxide interface. As a result, the trapped mobile ions may not completely de-trap and be fully detected during modest bias-temperature stressing.
In further discussion regarding mobile ionic charge, mobile ions are most commonly caused by impurity atoms, e.g. of sodium. Sources of sodium can be on quartz ware in oxide furnaces. Sodium may also be found in chemicals, such as, photoresists, being used during a semiconductor manufacturing process. For instance, if a photoresist is not totally removed during device fabrication, some sodium may remain and/or sodium may diffuse into an underlying oxide. Such may be the case when using a plasma stripping process to oxidize a photoresist. In this later case, the wafer would be subjected to an elevated temperature and further be subjected to an electric field, wherein sodium ions would tend to diffuse into the underlying oxide.
In U.S. Pat. No. 4,978,915, a method of manufacturing semiconductor devices involving the detection of impurities is disclosed. The method of the '915 patent increments a voltage on an MOS electrode and monitors a corresponding increment in charge, as measured by a coulombmeter. A voltage on the dielectric is forced using a probe contact. The resultant charge increments are measured via a probe contact to the coulombmeter. Such a method suffers from the above discussed disadvantages with respect to the use of MOS electrodes.
There is thus needed an alternative improved method and apparatus for obtaining "MOS-like" measurements. Such an apparatus, and method, should be well suited for providing desired "MOS-like" measurements and further having acceptable repeatability and accuracy of measurements suitable for advanced semiconductor monitoring needs.